Electronic apparatuses have been downsized and become denser in packaging. This market trend involves circuit-forming boards, to which electronic components are to be mounted, to be double-sided boards, or multilayer boards. As such, higher density boards that can accommodate more circuits and more components are developed. (e.g., Refer to “Surface Mounting Technique” January 1997, published by Daily Industry Newspaper Inc. in Japan. This edition includes a paper titled “Remarkable Development of Build-up Multi-layer PWB” written by Mr. Kiyoshi Takagi.)
Prior art is described hereinafter with reference to FIGS. 3A-3G and FIG. 4. Board material 1 shown in FIG. 3A is so called prepreg that is formed by the following method: Woven fabric, as a reinforcing member to be used in circuit boards, is impregnated with thermosetting epoxy resin and turned into a status called stage B. Board material 1 is coated with film 2 on both the upper and rear faces by a laminating method using a heated roll.
Next, as shown in FIG. 3B, via holes 3 are formed through board material 1 with a laser-beam, then via holes 3 are filled with conductive paste 4 produced by mixing conductive particles such as copper powder with thermosetting resin, curing agent, and solvent. Board material 1 thus becomes as shown in FIG. 3C. Films 2 are peeled off from both the faces, so that conductive paste 4 protrudes as shown in FIG. 3D. Above both the faces of material 1, copper foils 5 are prepared, and hot-pressed by a hot-presser (not shown), so that material 1 is thermally cured as shown in FIG. 3E, and conductive paste 4 is compressed, thereby connecting copper foil 5 on the upper face to that on the rear face. At this time, the epoxy resin impregnated in material 1 flows outside and forms flow-out section 6. Surplus portions on both the ends of material 1 are cut off, so that the shape shown in FIG. 3F is formed. Then copper foils 5 are processed into desired patterns by etching or other methods to form circuit 7, and finally, the double-sided circuit board as shown in FIG. 3G is obtained.
The foregoing method of manufacturing the circuit boards, however, sometimes results in instability of electrical connections with conductive paste between an upper-face circuit and a rear-face circuit of the circuit board as well as between an upper-face circuit and an inner circuit in the case of multi-layer circuit boards.
The inventors have analyzed circuit boards manufactured or made on a trial basis in order to investigate the causes of the instability discussed above. The results show that conductive particles in conductive paste 4 flow out from via holes 3 and become flow-out particles 8 as shown in FIG. 3E.
In other words, conductive paste 4 should be compressed vertically in FIG. 3 so that conductive particles in paste 4 contact with each other efficiently and firmly, and also contact with copper foil 5 firmly. However, in the process between FIG. 3D and FIG. 3E, flow-out section 6 is formed, so that thermosetting resin in board material 1 flows toward outer open ends during the heat and compression. At that time, the conductive particles in conductive paste 4 are pushed by the flowing thermosetting resin, and the particles flow laterally, as shown in FIGS. 3E-3G. As a result, conductive paste 4 is not efficiently compressed, so that the electrical connections with conductive paste 4 become unstable. The foregoing description refers to the board material made of glass woven fabric and thermosetting resin. However, failures are also observed when other materials are used, such as inorganic fiber other than glass, or organic fiber such as aramid fiber, or a reinforcing member made of non-woven fabric.
When woven fabric is used among others, flow resistance becomes small due to the structure using the woven fabric. Thus the thermosetting resin becomes so fluid that the conductive paste cannot achieve the electrical connection.
The inventors have carried out various experiments and analyzed areas around the via holes having insufficient electrical connections. The results show that flow-out particles as shown in FIG. 3E are observed more on the upper face than the rear (bottom) face of board material 1. The inventors have investigated the causes of this status, and reached the following conclusion.
In the case where prepreg employing the woven fabric made of glass fiber is used as the board material, the board material has the sectional view as shown in FIG. 4. In other words, board material 1 is manufactured by the following steps:
thermosetting resin such as varnish-state epoxy resin is impregnated as impregnating resin 10 into glass-fiber woven fabric 9;
resin 10 is rolled with a roller so that material 1 has a desired thickness; and
board material 1 is dried and turned into stage B.
Therefore, the thickness of board material 1 is a thickness of initial glass-fiber woven fabric 9 plus a thickness of impregnating resin 10 formed on the surface of woven fabric 9. The layer of impregnating resin 10 formed on the surface flows fiercely during the step of applying heat & pressing in manufacturing the circuit boards discussed previously. Then numerous flow-out particles occur around the surface of board material 1 and, as a result, connections between layers with conductive paste 4 become insufficient.
In the case where regular prepreg is used as the material of circuit boards, the prepreg is completely cured by heating and pressing, i.e., the prepreg is turned into stage C. Then the prepreg is pierced with a drill before being plated with a metal such as copper. As a result, layer-to-layer connection is achieved. Thus there is no such a problem as discussed above. Recently a manufacturing method of high-density circuit-forming boards has been developed. In this method, conductive material, namely, conductive paste is filled into via holes formed in board material in stage B so that connection between layers may be achieved. In this method the foregoing problem has occurred.
A variety of thicknesses are required design-wise to insulate respective layers of a circuit board from each other; however, the reinforcing members to be used in prepreg have a limited number of thicknesses. This present situation obliges some manufacturers to use the board materials with the impregnating resin 10, shown in FIG. 4, being in a layer that is rather thick.